Fix warning implicit conversion changes signedness

libusb/libusb.h:1368:12: warning: implicit conversion changes signedness: 'unsigned int' to 'int' [-Wsign-conversion]
        _packet = packet;
                ~ ^~~~~~
libusb/libusb.h:1373:67: warning: implicit conversion changes signedness: 'int' to 'unsigned int' [-Wsign-conversion]
        return transfer->buffer + (transfer->iso_packet_desc[0].length * _packet);
                                                                       ~ ^~~~~~~
Fix warning implicit conversion changes signedness

libusb/libusb.h:1328:12: warning: implicit conversion changes signedness: 'unsigned int' to 'int' [-Wsign-conversion]
        _packet = packet;
                ~ ^~~~~~
Fix warning implicit conversion changes signedness

libusb/io.c:1630:11: warning: implicit conversion changes signedness: 'unsigned int' to 'int' [-Wsign-conversion]
        r = ctx->pollfd_modify;
          ~ ~~~~~^~~~~~~~~~~~~
libusb/io.c:1658:11: warning: implicit conversion changes signedness: 'unsigned int' to 'int' [-Wsign-conversion]
        r = ctx->pollfd_modify;
          ~ ~~~~~^~~~~~~~~~~~~
Fix warning implicit conversion changes signedness

libusb/io.c:1539:11: warning: implicit conversion changes signedness: 'unsigned int' to 'int' [-Wsign-conversion]
        r = ctx->pollfd_modify;
          ~ ~~~~~^~~~~~~~~~~~~